The invention relates to a method for determining the reducing agent concentration (NH3) in the exhaust-gas flow of an internal combustion engine.
In piston combustion engines, it is necessary, between the shaft end of the gas exchange valve on the one hand and the valve drive (cam of the camshaft, valve actuating lever, or the like) acting on it on the other, to dispose a valve play compensation element in order to compensate for temperature-caused changes in the length of the valve shaft and changes, caused by wear to the valve seat, in the height of the shaft end when the gas exchange valve is closed, relative to the valve drive. To that end, a hydraulic valve play compensation element is used, which essentially comprises a cup-shaped cylinder and a piston guided in it; the cylinder interior can be subjected to pressurized oil, so that the two parts can be spread apart and can each come into contact without play on the shaft end of the valve on the one hand and the valve drive on the other. Via a throttle restriction, which is for instance provided by means of a defined gap between the cylinder wall and the piston, it is also possible during operation to compensate for a change in the height of the shaft end relative to the valve drive, whether it is caused by thermal expansion or wear to the valve seat, since via the outflow of oil through the throttle restriction, the total length of the valve play compensation element can be shortened. Such hydraulic valve play compensation elements have proven themselves over time and today are used in practically all piston combustion engines.
The disadvantage of the hydraulic valve play compensation element, however, is that an oil supply must be provided especially for it, which necessitates considerable engineering and production effort and expense at the cylinder head.
A further disadvantage is that any change in the viscosity of the oil used definitively affects the function of such a hydraulic valve play compensation, so that it is practically impossible to design one optimal cam shape for all operating states. Another disadvantage is the high oil consumption, with the result that the oil pump must be designed even for critical operating states, such as idling while hot, and hence is designed to be oversized for normal operation.
Mechanical play compensating elements are also known from European Patent Disclosure EP-A 0 032 284, German Patent Disclosure DE-A 36 07 170, and International Patent Disclosure WO 90/10787.